1. Field of the Invention
The present invention relates, generally, to multi-disk friction devices. More specifically, the present invention relates to a multi-disk friction device having improved lubrication characteristics.
2. Description of the Related Art
Multi-disk friction devices are employed in a wide range of applications as clutches or brakes. For example, such devices are frequently used in land-based vehicles. Generally speaking, such vehicles require three basic components: a power plant (such as an internal-combustion engine), a power-train, and wheels. The power-train's main component is typically referred to as the "transmission." Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle.
Transmissions include one or more gear sets, each of which may include an inner sun gear, intermediate planet gears supported by their carriers, and outer ring gears. Various components of the gear sets are held or powered to change the gear ratios in the transmission. The multi-disk pack-clutch is a friction device that is commonly employed as a holding mechanism in a transmission or differential. In addition, multi-disk friction devices also find use in industrial applications, such as wet brakes for braking the wheels on earth-moving equipment.
The multi-disk pack-clutch or brake assembly includes a plurality of ring-shaped disks and has a clutch sub-assembly. The sub-assembly includes a set of plates arranged in a torsionally rigid manner and fixed against displacement on a disk carrier as well as a set of friction disks interleaved between one another and fixed against displacement on a hub. The clutch or brake assembly also typically includes a piston. When a component of a gear set is to be held, as during a particular gear range, a piston is actuated so as to cause the plates and friction disks to come in contact with respect to one another. The plates mutually engage in a gearing manner, and the friction disks mutually engage in a gearing manner. The sets are mutually displaceable relative to each other in an axial direction and can be brought into and out of engagement in pairs. In "open pack" operation, the plates and friction disks normally turn past one another without contact. In certain applications, it is known to employ several multi-disk friction devices in combination to establish different drive connections throughout the transmission or differential to provide various gear ratios in operation or to brake a component.
Multi-disk clutches or multi-disk brakes having disks which are interconnected, i.e., made in a single piece from frictional material are also known in the related art. Examples of such devices are disclosed in DE 31 49 880 C2, DE 35 32 759 C1, and DE 31 18 565 A1.
Each friction disk includes a carrier plate made of, for instance, steel. The carrier plate includes a friction surface on at least one annular face thereof. The friction surface generally includes a fiber-material manufactured from paper or a paper-like material. The structure of the plate surfaces is generally smooth. The confronting faces of the interleaved plates and friction disks are, therefore, covered with frictional surfaces. WO 97/32678 discloses a special plate-surface structure made of steel used to improve the coefficient of friction in the pairings of the friction disks and plates.
When a friction device is engaged, kinetic energy is converted into thermal energy and a considerable amount of heat is generated. If the frictional surfaces get too hot, they can burn, which damages the friction surfaces and degrades the clutch's or brake's operational effectiveness. Additionally, the requirements placed on the service life of multi-disk clutches and similar units are exceptionally high. The frictional surfaces must be able to sustain a high, but undetermined, number of engagements without changing the power-transmission behavior, e.g., by wear and tear or lack of heat discharge.
As such, in the power-transmission units discussed above, lubrication of the individual components plays an important role. It must be ensured, in particular, that lubricating oil reaches the frictional surfaces of the disks. For this purpose, lubricating oil is typically sprayed onto the frictional surfaces.
However, during operation of the multi-disk clutch, a situation can occur in which a friction disk is deflected from its axially perpendicular position and performs a kind of wobbling motion. In this event, the friction disk assumes a certain minimal inclination toward the axially perpendicular direction. The consequence is that the portion of the edge of the friction disk that is deflected from the axially perpendicular position rests on the plate with which it cooperates, thus, causing a local area of high wear. This leads to a wearing off or removing of some of the oil film from the friction plate. Furthermore, a portion of the friction surface "drying out," may experience rapid localized wear and tear by overheating, and, ultimately, a complete breakdown of the friction disk and/or hot spots on the friction surface. This phenomenon occurs mainly when the clutch is disengaged, and not so frequently when the clutch is engaged.
It is known in the related art to provide the frictional surfaces with a porous, sponge-like structural arrangement, among others, such that a certain amount of oil storage is achieved. The stored oil is supplied to the frictional surfaces when the clutch is applied. It has been noticed, however, that in some cases, wear and tear will occur without the disks taking a decisive part in the power transmission. The breakdown of one of the disks will lead to the failure of the entire unit.
Accordingly, there remains a need in the art for a power-transmission unit, in particular, a multi-disk friction device such as a clutch or brake, the service life of which, specifically in "open pack" mode, is further increased as compared with known units and breakdowns of the disks of such units are avoided.